(1) Field of the Invention
The present invention relates to a one-component type developer for electrophotography. More particularly, the invention relates to an electrically conductive magnetic developer capable of development without the use of a magnetic carrier, in which fixing and electric resistance-controlling fine particles are tightly held on the surfaces of fixing magneto-sensitive particles in the substantially non-embedded state, whereby low electric resistance and good flowability, desireable for the development operation, can be maintained and contamination of the background or parts of a copying machine with the above-mentioned fine particles separated from the surfaces of the fixing magneto-sensitive particles can be drastically diminished.
(2) Description of the Prior Art
As one of methods for developing electrostatic latent images formed by electrophotography, there has heretofore been broadly adopted a so-called magnetic brush method. According to the basic magnetic brush method, toner particles comprising a pigment and a resin imparting a fixing property and desirable electric characteristics to the pigment are mixed with a magnetic carrier such as iron powder to form a developer, and an electrostatic latent image-carrying surface of a support is tightly contacted with magnetic brushes composed of this developer to visualize the latent image with the toner particles. In this method using such mixture of a toner and a magnetic carrier, only the toner particles are consumed by the development operation and the mixing ratio of the toner to the magnetic carrier is changed. Accordingly, there must be conducted a troublesome operation of supplying the toner into a developing mechanism to maintain a predetermined mixing ratio between the toner and magnetic carrier.
A so-called magnetic developer as disclosed in the specification of U.S. Pat. No. 3,639,245 or Japanese Patent Application Laid-Open Specification No. 20729/75 is known as a developer capable of development with a toner alone without the use of a particular carrier. A customarily used magnetic developer of this type is ordinarily formed by dispersing a powder of a magnetic material such as triiron tetroxide, optionally with other pigment, into a binder resin medium, forming the resulting composition into particles and embedding an electrically conductive substance such as carbon black into the surfaces of the particles to impart to the entire particles a property of being magnetically attracted and to render the surfaces of the particles electrically conductive.
This magnetic developer has an advantage that a clear and sharp image with reduced edge effects can be obtained according to the magnetic brush development method without using a magnetic carrier or the like. However, preparation of this magnetic developer involves various defects and difficulties.
Further, such known magnetic developers are still insufficient with respect to the flowability of developer particles, and because of this insufficient flowability, various troubles are caused at the developing step. More specifically, since the known magnetic developer has not a flowability sufficient to apply the developer uniformly onto a developing roller (sleeve), for example, masses of the developer are formed on the surface of the sleeve and the background is often contaminated when such masses of the developer particles fall down on a copying sheet or the like. Further, blurred images are readily formed because of non-uniform adhesion of the developer particles to the sleeve surface.
As means for improving the flowability of particles of a magnetic developer, there is known a method in which finely divided silica is incorporated as a lubricating agent in the magnetic developer particles. Since the electric resistance of the silica powder adhering around the particles of the electrically conductive magnetic developer is relatively high, the electric resistance of the developer as a whole is increased and such defects as blurring of image contours are caused.
We previously found that when a composition formed by dispersing a powder of magnetic material such as triiron tetroxide, optionally with other pigment, into a binder resin medium is shaped into spherical particles having fine convexities and concavities on the surface thereof or having a crater-like rough surface and such spherical particles are dry-blended with flowability- and electric resistance-controlling fine particles composed of carbon black or the like to form a developer, said fine particles are held on the surfaces of the spherical particles and low electric resistance and high flowability, desirable for the developing operation, can be attained in the resulting developer, and that although said fine particles are present on the surfaces of spherical particles in the non-embedded state, isolation of the fine particles from the surfaces of the spherical particles is substantially prevented and contamination of the background is not substantially caused. Based on this finding, we proposed this excellent developer (see Japanese Patent Application Laid-Open Specification No. 88277/76) corresponding to U.S. Application Ser. No. 732,759.
However, in our previously proposed developer, fine particles such as of carbon black are not completely fixed to the surfaces of fixing magneto-sensitive spherical particles. Accordingly, if the dry blending of both the particles is completed in a short time, a large quantity of carbon black is incorporated so as to reduce the electric resistance or if the number of convexities and concavities on the surfaces of the magneto-sensitive particles is small, at the step of agitating the developer in a developer tank or the step of the magnetic brush development, carbon black is often isolated from the surfaces of fixing magneto-sensitive particles and scattered to contaminate an optical system of a copying machine or the background of a copying sheet. This isolation of carbon black particles is especially conspicuous when fixing magneto-sensitive particles are those prepared by kneading a fine powder of a magnetic material with a melt of a resin binder, pulverizing the mixture under cooling and classifying the resulting particles by sieving. In this case, the fixing magneto-sensitive particles have none of convexities or concavities on the surfaces thereof or if they have, the number of convexities and concavities is very small. Further, since the fine powder of the magnetic material is covered and wrapped with the binder resin, the fixing magneto-sensitive particles have a relatively high electric resistance. Therefore, these particles should be dry-blended with a large quantity of carbon black. For these reasons, isolation of carbon black becomes especially conspicuous in case of the above-mentioned fixing magneto-sensitive particles.
In the above-mentioned one-component type developer formed by dry-blending fixing magneto-sensitive particles with carbon black, as the mixing ratio of carbon black increases, the tendency that carbon black particles separate from a fixed image when the fixed image is rubbed with a finger or the like becomes conspicuous and the fixed image is readily blurred. In short, this one-component type developer is still insufficient in the friction resistance of a fixed image.